1. Field of the Invention
The present invention relates to a power steering apparatus for reducing steering effort by a driver which is required to steer the steering wheel of a vehicle, and more particularly to a power steering apparatus in which an electric motor and a hydraulic pump are used to reduce the energy consumed by the power steering apparatus.
2. Discussion of the Prior Art
Both of hydraulic type power steering apparatus and electric type power steering apparatus are conventionally used. In most of the hydraulic type power steering apparatus, the hydraulic pump is driven by an engine of a vehicle. In such apparatus, the hydraulic pump is always driven by the engine even when the vehicle continues to travel straight during which any assistance for steering is not required. Such hydraulic type power steering apparatus therefore has a problem that the energy loss is large.
In the electric type power steering apparatus, the hydraulic pump can be omitted, because assist force is generated directly by an electric motor. In such system, the electric motor is activated only when the assist is necessary. Therefore, the electric type power steering apparatus has an advantage that the energy loss is smaller than the hydraulic type power steering apparatus.
However, comparing the hydraulic type and the electric type, the hydraulic type has a higher reliability, because the direction of an assist force is mechanically determined.
In the electric power steering apparatus, many measures have been taken to improve its reliability, examples of which include the addition of a safety circuit for preventing a runaway of the control device, and detection means for detecting mechanical abnormality. However, there has been a demand to improve the reliability of the power steering apparatus much more.
To satisfy the demand, the hybrid type power steering apparatus has been proposed in which a hydraulic pump is driven not by an engine of a vehicle but by an electric motor. The rotational speed of the motor is controlled, for example, in accordance with traveling conditions of the vehicle, whereby the amount of the operation fluid supplied to the assist force generating mechanism is varied.
Although the hybrid type power steering apparatus has a higher reliability, the energy loss is still larger compared to the electric type power steering apparatus, because the hydraulic pump is not stopped even when the engine is in idle condition or when the vehicle continues to travel straight. If the pump were stopped in such conditions, the pump would not be able to quickly output pressurized fluid after the rotation of the pump is resumed. This would produce a lag in the assist operation.
Further, in the hybrid type power steering apparatus, the pump unit including a hydraulic pump and a electric motor for driving the pump is mounted apart from the assist force generating mechanism. In such apparatus, it is necessary to use long hydraulic pipes to connect the pump unit with the assist force generating mechanism. As a result, a large space is required to install the hydraulic pipes. Since the connection of the hydraulic pipes is carried out after the pump unit and the assist force generating mechanism are assembled in the vehicle, work for connecting the hydraulic pipes must be carried out in a small space. This deteriorates the working efficiency.